Quadrature Phase Shift Keying (QPSK) data modulation is used to increase the data rate capability over Binary Phase Shift Keying (BPSK) data modulation. To improve data performance in multi-path channel conditions and to reduce the transmit power spectral density, direct sequence spreading is applied to the data modulation. Differential data detection is performed to simplify the demodulation process, resulting in differential QPSK (DQPSK) reception. The existing 802.11b waveform provides both DBPSK and DQPSK data modulation using a BPSK signal for the direct sequence spreading to provide 1 and 2 Mbps data capability.
To achieve the 1 and 2 Mbps data rates, 11 chips are used to spread the data modulated signal. An 11 chip Barker sequence is used for the spreading sequence. The 11 chip Barker sequence possesses excellent autocorrelation properties, providing a maximum correlation sidelobe level of 1/11 the peak correlation value. To achieve this excellent correlation property on each data symbol, the same 11 chip Barker sequence is used to spread each data symbol.
As an alternative to using short repeated sequences, BPSK modulation may be used to spread the data. BPSK provides a simple straight forward means to spread either the BPSK or QPSK data. To meet the 802.11 spectral requirements, the BPSK spread signal is passed through a lowpass filter to reduce the power spectrum sidelobe level. The filtered BPSK signal is operated within the linear region of the power amplifier to minimize spectral regrowth output from the RF power amplifier.
There are, however, some limitations to using the aforementioned techniques. First, waveforms using short spreading sequences, such as the 11 chip Barker sequence used for 802.11b waveforms, limit the delay spread range for channel multi-path equalization, because two adjacent symbols can be opposite in polarity. Further, short, repeated, spreading sequences also enable unauthorized listeners to easily recover the data symbol stream. Longer sequences remove these limitations. However, longer spreading sequences do not provide excellent autocorrelation properties across short sections (11 chips for the 802.11b waveforms) of the spreading sequence. Degradation in the autocorrelation property directly degrades the bit-error-rate (BER) system performance.
Second, BPSK spreading waveforms limit power efficiency at the RF power amplifier, because they require the amplifier to operate in a linear mode to prevent spectral sidelobe regrowth. Spreading data using constant envelope modulation signals, like Minimum Shift Keying (MSK) or near constant envelope modulation, like Quasi-bandlimited MSK (QBL-MSK) and Raised Cosine filtered Offset Quadrature Phase Shift Keying (RC-OQPSK), however, enable the RF power amplifier to operate in the nonlinear mode, increasing power efficiency.
Standard parallel demodulation techniques for MSK, QBL-MSK, and RC-OQPSK despread the signal using independent I and Q sequences, and require two orthogonal or near orthogonal spreading sequences. Gold codes are typically used because of their good autocorrelation and cross-correlation properties. However, Gold codes also require, at minimum, 31 chips (lowest length Gold code) of spreading on both the I and Q data, and increasing the number of chips results in a reduced data rate for the same operational chip rate. To reduce the number of spreading chips required for these constant or near constant envelope modulation signals, serial formatting is applied to the spreading waveform. Serial formatting combined with serial demodulation enables these waveforms to be demodulated similarly to BPSK.
For a serial despread MSK, QBL-MSK, or RC-OQPSK signal, the repeating 11 chip Barker sequence can be used for the spreading sequence. Autocorrelation properties for the 11 chip Barker sequence are excellent, providing suppression of the undesired serial demodulation term. To avoid the limitations associated with the short spreading sequence, a longer spreading sequence is used. As described previously, longer spreading sequences do not provide excellent autocorrelation properties across short sections (11 chips for the 802.11b waveforms) of the spreading sequence. The poor autocorrelation properties associated with the long spreading sequence result in the undesired serial demodulation term not being suppressed.
A BER performance curve with a maximum of a quarter chip timing error (sampling at twice the chip rate) for DQPSK data modulations with QBL-MSK spreading for a short 8 chip Neuman-Hoffman sequence (00001101) is shown in FIG. 1. As depicted in FIG. 1, for ideal timing (0 or 0.5 Tc), a 10−6 BER is achieved at approximately Es/No equal to 11.9 dB, while the maximum Tc/4 timing error condition requires the Es/No to increase to approximately 12.5 dB to provide the same bit error rate.
The BER performance curve with a maximum of a quarter chip timing error (sampling at twice the chip rate) for DQPSK data modulations with QBL-MSK spreading for a long, random spreading sequence is shown in FIG. 2. As depicted in FIG. 2, for ideal timing (0 or 0.5 Tc), a 10−6 BER is achieved at approximately an Es/No equal to 12.5 dB, while the maximum Tc/4 timing error condition requires the Es/No to increase to approximately 16 dB to provide the same bit error rate. For ideal timing, the additional Es/No required for the long sequence versus the short sequence is only 0.6 dB. For the maximum Tc/4 timing error condition, the additional Es/No required for the long sequence versus the short sequence is 3.5 dB. This significant degradation in BER performance for timing error must be reduced by either increasing the timing resolution or by compensating for the poorer autocorrelation properties of the long spreading sequence over the shorter symbol spreading length. Increasing the timing resolution requires an increase in the sampling rate, which increases the demodulator complexity and DC power consumption.
To minimize demodulator complexity and power consumption, the present invention provides a compensation approach, among other features.